A battery is an electrochemical device that generates electric current by converting chemical energy to electrical energy. Its essential components are: positive and negative electrodes made of more or less electrically conductive materials; a separating medium (e.g., a separator); and an electrolyte. There are several types of batteries. Two types of batteries are: primary (or non-reversible or disposable), and secondary (or reversible or rechargeable). Another way of characterizing batteries is by the nature of the electrolyte; for example, the electrolyte may be aqueous or non-aqueous (e.g., organic). The nature of the electrolyte can have a material impact on the selection of the separator.
Batteries with aqueous electrolytes include, but are not limited to: alkaline batteries, nickel metal hydride (NiMH) batteries; nickel cadmium (NiCd) batteries; and zinc-air batteries. Separators for these batteries must be hydrophilic. Hydrophilic refers to the ability to `wet out` the electrolyte. `Wet-out` refers to the ability to cause a materials (e.g., electrolyte) to penetrate more easily into, or spread over the surface of another material (e.g., separator).
In primary, aqueous electrolyte batteries, nylon and hydrophilically-treated polyolefin separators may be used. Nylon is an inherently hydrophilic material, and polyolefin, an inherently hydrophobic material, must be treated with a wetting agent prior to use to render it hydrophilic.
In secondary, aqueous electrolyte batteries, nylon and hydrophilically treated polyolefin separators have been used, but both are deficient because they degrade (i.e., the nylon or the hydrophilic treatment on the polyolefin) over the life of the rechargeable battery. The degradation can shorten the life of that battery.
Accordingly, there is a need for a new battery separator that is hydrophilic and resists degradation.
U.S. Pat. No. 4,309,494 discloses a battery separator made from an ethylene vinyl alcohol (or EVOH) copolymer, but this separator is made by a solvent extraction process, or by a swelling, in the presence of the electrolyte, of the nonporous membrane and is not oriented. U.S. Pat. No. 5,238,735 discloses a battery separator made from a polyolefin by a particle stretch technique. In the particle stretch technique, synthetic resin particles are added to the polyolefin, so that when the nonporous film is stretched, `peeling` will be induced at the interface of the particles and polyolefin to create pores. Japanese Kokai 56/049157 discloses an ethylene vinyl alcohol copolymer porous film for blood-plasma separation made by a solvent extraction process and not by an orientation process. WO97/20885 discloses a microporous ethylene-vinyl alcohol copolymer membrane that may be used as a separator. The membrane is made by a `phase separation` technique. In the phase separation technique, the EVOH copolymer is dissolved into a compatible polymer or compound, and the non-EVOH material is removed. Phase separation is not orientation.
Accordingly, there is a need for a new battery separator that is permanently hydrophilic, resists degradation, and is simply and economically produced.